Age of uranium mineralization at the Highland Mine, Powder River basin, Wyoming, as indicated by U-Pb isotope analyses

Abstract

Most of the uranium deposits in the Powder River basin occur at various stratigraphic levels in the Wasatch Formation of Eocene age, but pollen and spores found in the Highland mine host rocks indicate that the deposits here may be in the upper part of the Fort Union Formation of Paleocene age. Analyses of U-Pb isotopes in several samples of ore from the Highland mine were made in order to compare the apparent age of mineralization of this deposit with that of host rocks of equivalent age at Gas Hills, Crooks Gap, and Shirley basin, Wyoming. The samples used for age determinations were collected in the southernmost of a series of open pit mines and in the uppermost of the three host units. All samples contain only reduced uranium minerals which are mainly coprecipitated mixtures of pitchblende and coffinite. Sample EP17-7 consists of ore at the boundary with oxidized yellow sandstone and sample EP17-8 of ore 6 m (20 ft) from the interface with oxidized sandstone. Sample J represents calcite-cemented ore from a thin zone at the base of the upper host sandstone unit (lower limb ore). The others are randomly selected grab samples of mineralized material in the oremore » deposit at the same level as EP-17-7. With the exception of sample J, all samples were collected from exposures of ore on the pit floor 60 m (200 ft) below the original land surface and at an elevation of 5,060 feet. Sample J came from the pit floor at an elevation of about 5,040 ft.« less